Sonny Bill Williams conceded he tried to do too much in the Kiwis' 34-2 Rugby League World Cup final loss to Australia in Manchester this morning.

It was almost a given that New Zealand needed Williams to have a big game if they were going to stand any chance of beating the Kangaroos and defending their title.

But the recently crowned international player of the year had a rare off night and wasn't able to make a big impression in the game.

Speaking afterwards, Williams said he was devastated that the Kiwis lost.

"I'm heartbroken, I'm really disappointed," Williams said.

"I left it all out there today. That last 20 minutes was disappointing.

"I tried to overplay my hand and try to come up with something special and it wasn't to be.

"I definitely feel bad for the brothers but full credit to Australia. They were just too good today."

Williams has no regrets about deciding to play in the World Cup, rather than take a break, which was his initial intention and he said it was a hugely memorable experience for him to play for the Kiwis again.

"It's the thing that I missed," he said.

"I felt it after a couple of weeks in camp. It's just like a family, a brotherhood, and it's something you can't really get when you are playing at club level.

"But I'm really disappointed right now and disappointed for the boys. I just couldn't come up with it today. I tried my arse off but it wasn't meant to be.

"In that last 20 minutes, we all tried, and me the most," he added.

"We tried to push a pass and it just wasn't meant to be. Australia really deserved it and played well."

If the Kiwis had triumphed, Williams would have cemented his plaice in the history books as being the only player to have won a rugby league and rugby union World Cup. However, Williams said that wasn't what had been his motivation.

"It was never about that. It was just about having no regrets when I retire," he said.

"You win some, you lose some. It's been a privilege to put the Kiwis jersey back on. Just really disappointed for the fans back home and our families."